Servo control of a disk drive

ABSTRACT

A disk drive comprising a head for accessing a disk, an actuator for moving the head in the radial direction of the disk and a controller. The controller for servo control of the actuator based on servo data read out from the disk by the head. The controller is configured for maintaining a width of the adaptive chasing notch filter below a specified value in servo system control and estimating an oscillation frequency of the actuator.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from the Japanese Patent ApplicationNo. 2009-206491, filed Sep. 7, 2009 the disclosure of which isincorporated herein in its entirety by reference.

BACKGROUND

Devices using various types of disks such as optical disks,magneto-optical disks, and flexible magnetic disks are known as diskdrives. Of these, the hard disk drive (HDD) is used in many electronicdevices, such as video recording and playback devices and car navigationsystems, in addition to computer systems.

An HDD has a swinging actuator and a head slider supported by theactuator. The actuator is driven by a voice coil motor (VCM). Usually,the actuator has several mechanical resonance modes. Consequently, theservo system becomes unstable because of these modes, and mechanicaloscillations occur at the characteristic frequencies (resonancefrequencies).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically depicting the HDD, in accordanceto an embodiment of the present invention.

FIG. 2 is a block diagram modeling the servo control system in the HDD,in accordance to an embodiment of the present invention.

FIG. 3 is a diagram showing the function which represents the adaptivechasing notch filter and the update quantity of the center frequency, inaccordance to an embodiment of the present invention.

FIG. 4 is a flow chart showing the flow of the adaptation process of theadaptive chasing notch filter, in accordance to an embodiment of thepresent invention.

FIG. 5 is a flow chart showing the method for adjusting the centerfrequency of the adaptive chasing notch filter, in accordance to anembodiment of the present invention.

FIG. 6 is a flow chart showing the method for determining the width ofthe adaptive chasing notch filter, in accordance to an embodiment of thepresent invention.

The drawings referred to in this description should be understood as notbeing drawn to scale except if specifically noted.

DESCRIPTION OF EMBODIMENTS

Reference will now be made in detail to embodiments of the presenttechnology, examples of which are illustrated in the accompanyingdrawings. While the technology will be described in conjunction withvarious embodiment(s), it will be understood that they are not intendedto limit the present technology to these embodiments. On the contrary,the present technology is intended to cover alternatives, modificationsand equivalents, which may be included within the spirit and scope ofthe various embodiments as defined by the appended claims.

Furthermore, in the following description of embodiments, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present technology. However, the present technologymay be practiced without these specific details. In other instances,well known methods, procedures, components, and circuits have not beendescribed in detail as not to unnecessarily obscure aspects of thepresent embodiments.

A magnetic disk used in an HDD has a plurality of data tracks and aplurality of servo tracks formed in concentric circular shapes. Eachservo track is comprised of a plurality of servo sectors containingaddress information. In addition, each data track is comprised of aplurality of data sectors containing user data. The data sectors arerecorded between servo sectors which are separated in thecircumferential direction.

An HDD has a swinging actuator and a head slider supported by theactuator. The HDD reads out the address information of the servo sectorsby the head slider and controls the actuator in accordance with theaddress information (servo control). Thus, the HDD can move (seek) thehead slider to the desired radial position (target data track) and canposition at that location (following). The head slider positioned at thetarget data track writes data to or reads data from the target datasector in the track.

The actuator is driven by a voice coil motor (VCM). Usually, theactuator has several mechanical resonance modes. Consequently, the servosystem becomes unstable because of these modes, and mechanicaloscillations occur at the characteristic frequencies (resonancefrequencies).

Therefore, a conventional HDD uses a notch filter in the servo controlof the head slider. The notch filter is inserted on the output side ofthe controller in the servo loop of the head slider and has a centerfrequency which is the same as a resonance frequency of the actuator.The notch filter reduces the servo gain at the oscillation frequencyincluded in the control signal to stabilize the servo control of thehead slider.

Accompanying the increase in the tracks per inch (TPI) in an HDD, anincrease in the servo band is sought in order to maintain thepositioning precision of the head. One factor impeding the increase inthe servo band is the consumption of the phase margin by the phase delaycaused by the notch filter. If the consumption of the phase margin canbe lowered, the servo band can be increased.

However, conventionally, a notch filter is designed so that consumptionof the phase margin is limited, and the oscillation problem is notproduced in a servo system under a variety of conditions such asvariations in the mechanical parts or temperature fluctuations. Theconsumption of the phase margin is difficult to further reduce withoutincreasing the risk of servo system oscillations. Therefore, smallervariations of the mechanical parts and optimally setting each servosystem individually at the manufacturing plant are expected. However,both increase costs, and simpler methods are sought.

On the other hand, a phenomenon was discovered where oscillations werecaused by the method for securing the HDD in the system. Theseoscillations did not occur in the tests during manufacturing or in asingle HDD unit because the oscillations began when installed in thesystem. Therefore, only measures which preventatively added notchfilters could be adopted; the consumption of the phase margin increased;and the servo band became more difficult to increase.

Therefore, the next proposed technology estimates the frequency duringoscillation and generates the notch filter required to stop theoscillation. Thus, the notch filter which is used preventatively againstmechanical oscillations having a low probability of oscillation can beomitted, or the filter width can be narrowed. In addition, theconsumption of the phase margin can be suppressed resulting in anincrease in the servo band.

Some technologies use the technique of a frequency-following peak filter(adaptive chasing notch filter). The peak filter is positioned in theservo loop parallel to a main controller and acts to suppress theoscillations of the actuator (head slider) caused by externaloscillations.

The external oscillation frequency can be estimated by using thedenominator of the peak filter. The adaptive chasing notch filterestimates the external oscillation frequency, and gradually moves thepeak frequency closer to the external oscillation frequency. Thedenominator of a peak filter has the same shape as the numerator of anotch filter. Consequently, even if the numerator of the notch filter isused, the external oscillation frequency can be similarly estimated.

However, the object of the suppression by the notch filter is not theactuator oscillations caused by external oscillations, but theoscillations caused by the actuator oscillations. It was found that aconventional implementation method of an adaptive chasing notch filtercannot be applied without modification to the frequency-following notchfilter (adaptive chasing notch filter) because of the difference betweenexternal oscillations and actuator oscillations.

The adaptive chasing peak filter described in the present technologiesestimates the frequency of the external oscillations and updates thepeak frequency so that the peak frequency approaches the oscillationfrequency. By repeating this process, the peak frequency of the adaptivechasing peak filter approaches and finally matches the externaloscillation frequency (actuator oscillation caused by externaloscillation). The suppression target of the adaptive chasing peak filteris the oscillation of the actuator (head slider) caused by the externaloscillation. Even if the peak frequency of the peak filter matches theexternal oscillation frequency, the actuator oscillation caused by theexternal oscillation is not completely stopped. Consequently, the peakfrequency can be made to gradually approach the external oscillationfrequency and converge at that value.

On the other hand, the adaptive chasing notch filter estimates theoscillation frequency of the actuator and gradually moves the centerfrequency of the notch filter closer to the oscillation frequency. Incontrast to external oscillations, the oscillations caused by actuatoroscillations are eliminated when the oscillation conditions are removed.Therefore, before the oscillation frequency is correctly estimated(before the center frequency of the notch filter matches the oscillationfrequency), the oscillations stop when the oscillation conditions areremoved by the skirt of the notch filter.

The oscillation frequency cannot be estimated after the oscillationsstop, and the optimal notch filter cannot be generated. In addition,when oscillations caused by the actuator oscillations do not exist, theproblem is that the frequency is estimated for smaller oscillations; thecenter frequency of the notch filter moves; and the oscillations resume.

The disk drive in an embodiment of the present invention has a head foraccessing the disk, an actuator for moving the head in the radialdirection of the disk, and a controller for conducting the servo controlof the actuator by using the servo data read out from the disk by thehead. While maintaining the width of the adaptive chasing notch filterwhich performs the filter computations in the servo control system belowthe specified value, the controller references the internal variables ofthe adaptive chasing notch filter, and repeatedly estimates theoscillation frequency of the actuator and updates the center frequencyin response to the estimation result. Furthermore, the controllerincreases the width after the center frequency converges at theoscillation frequency. Thus, the difference between the oscillationfrequency of the actuator and the center frequency of the notch filtercan be set in the desired range and the oscillations of the actuator canbe effectively suppressed.

In one embodiment, the specified value is 0. Thus, the center frequencyof the notch filter can approach the oscillation frequency of theactuator more appropriately.

In another embodiment, the controller repeatedly estimates and updatesthe center frequency in response to the estimation result to graduallymove the center frequency closer to the oscillation frequency, changesthe magnitude relationship between the center frequency and theoscillation frequency, and ends the estimation of the oscillationfrequency and the update of the center frequency. Thus, the differencebetween the oscillation frequency of the actuator and the centerfrequency of the notch filter can be set in the desired range by asimple structure.

In another embodiment, the controller repeatedly estimates and updatesthe center frequency in response to the estimation result and increasesthe update quantity of the center frequency in response to the number ofconsecutive updates having the same sign for the center frequency. Thus,the convergence speed of the center frequency can be accelerated.

In a further embodiment, the maximum value of the absolute value of theupdate quantity of the center frequency is specified. Thus, the centerfrequency is not too much larger than the oscillation frequency.

In yet another embodiment, the controller compares the magnitude of thevalue in the estimation calculation of the oscillation frequency to athreshold. If the value is greater than the threshold, the estimation ofthe oscillation frequency and the update of the center frequency areexecuted. Thus, the effects of noise can be reduced.

In a further embodiment, the controller uses a function value whichrepresents the oscillations of the actuator with the position errorsignal set as the variable in order to judge whether the oscillations ofthe actuator have stopped, and determines the width of the adaptivechasing notch filter based on the value of the width when theoscillations stop. Thus, an appropriate width for stopping theoscillations can be set.

In another embodiment, the controller determines a value Wmin greaterthan the value of the width for stopping the oscillations for the widthof the adaptive chasing notch filter. Thus, oscillations can beprevented more reliably.

In one embodiment, in the judgment about the stopping of theoscillations, the controller compares the function value and a firstthreshold to judge whether the oscillations have stopped, and increasesthe width when the function value is between a second threshold which issmaller than the first threshold and the first threshold, and when thevalue of the width is less than the value Wmin. Thus, the margin can bemaintained in the notch filter.

In one embodiment, when the function value is less than a thirdthreshold which is less than the second threshold, and the width isgreater than Wmin, the controller decreases the width. Thus, theactuator oscillations are more reliably prevented.

In one embodiment, the maximum value of the width is specified, and thecontroller determines the width at a value less than the maximum value.Thus, the servo system is prevented from becoming unstable.

In another embodiment, the maximum value is represented by a function ofthe center frequency. Thus, the servo system can be more reliablyprevented from becoming unstable.

In a further embodiment, the controller uses the filter coefficientsdetermined by the adaptation of the adaptive chasing notch filter to seta new fixed notch filter. Thus, the fixed notch filter can beappropriately set.

In another embodiment, the controller references the internalcoefficients at the single sampling rate timing, and the adaptivechasing notch filter operates at the multiple sampling rate. Thus, inthis simple structure, oscillations can be suppressed by the adaptivechasing notch filter.

In one embodiment, the controller calculates the difference of values ofthe same internal variable at different sampling times in the frequencyestimation and removes the DC components of the internal variables.Thus, the DC components in the internal variables can be removed and theoscillation frequency of the actuator can be appropriately estimated bya simple structure.

An embodiment of the present invention is described below. To clarifythe description, the descriptions and drawings are omitted or simplifiedwhen appropriate. In each drawing, the same reference numbers areassigned to the same elements. To clarify the descriptions, duplicatedescriptions are omitted as needed. The embodiment of the presentinvention is described below for a hard disk drive (HDD), which is anexample of a disk drive.

In various embodiments, the HDD has a notch filter with a variablecenter frequency in the servo control system (servo loop). The centerfrequency of the notch filter is the frequency having the smallest gain,and the filter shape is symmetric or asymmetric with respect to thecenter frequency. The HDD estimates the oscillation frequency of theactuator and moves the center frequency of the notch filter closer tothat frequency. The HDD repeats the estimation of the oscillationfrequency and the update of the center frequency, and sets the centerfrequency in the desired range from the oscillation frequency. Thus, thenotch filter in the embodiment is a frequency-following adaptive notchfilter (adaptive chasing notch filter).

FIG. 1, HDD 1 has a magnetic disk 11, which is the disk for storingdata, in an enclosure 10. A spindle motor (SPM) 14 rotates the magneticdisk 11 at the specified angular velocity. A head slider 12 foraccessing the magnetic disk 11 is set up for each recording surface ofthe magnetic disk 11. Access is the high-level concept of reading andwriting.

Each head slider 12 is provided with a slider which flies above themagnetic disk and a head element which is secured to the slider andconverts between magnetic signals and electrical signals. Each headslider 12 is secured to the tip of an actuator 16. The actuator 16 isconnected to a voice coil motor (VCM) 15 and swings about the swingingshaft at the center to move the head slider 12 in the radial directionabove the rotating magnetic disk 11.

The actuator 16 has a voice coil on the side opposite the head slider 12and sandwiches the swinging shaft, and the voice coil constitutes a partof the VCM 15. The drive force is applied to the voice coil by the drivecurrent flowing in the voice coil in the magnetic field from a magnet. Aramp 17 is fixed inside of the enclosure 10 near the outer peripheraledge of the magnetic disk 11. When the power supply of the HDD 1 is offor idle, the actuator 16 stops on the ramp 17 on the outer side of themagnetic disk 11. Typically, the front projection of the actuator 16slides on the ramp 17. In one embodiment, this can be applied to acontact start and stop (CSS) HDD in which the actuator 16 lands on thespecified area in the magnetic disk 11 and stops.

Circuit elements are mounted on a circuit board 20 secured to the outerside of the enclosure 10. A motor driver unit 22 drives spindle motor(SPM) 14 and the VCM 15 in accordance with the control data from anHDC/MPU 23. A RAM 24 functions as a buffer for temporarily storing theread data and the write data. Arm electronics (AE) 13 in the enclosure10 select the head slider 12 to access the magnetic disk 11 from aplurality of head sliders 12, amplifies the playback signal, andtransmits the signal to a read/write channel (RW channel) 21. Inaddition, the recording signal from the RW channel 21 is transmitted tothe selected head slider 12. The present invention can be applied to anHDD having only one head slider 12.

In a read process, the RW channel 21 amplifies the read signal suppliedfrom the AE 13 to a constant amplitude, extracts the data from theacquired read signal, and decodes the data. The read-out data includesuser data and servo data. The decoded read user data and servo data aretransmitted to the motor drive unit (HDC/MPU) 23. In addition, in awrite process, the RW channel 21 code modulates the write datatransmitted from the HDC/MPU 23, transforms the code-modulated writedata into a write signal, and supplies the signal to the AE 13.

The HDC/MPU 23, which is the controller, controls the required processesrelated to data processing, such as read/write process control, commandexecution order management, positioning control (servo control) of thehead slider 12 (actuator 16) using the servo data, interface controlwith a host 51, defect management, and error handling process when anerror is generated, and the overall control of the HDD 1.

In one embodiment, the HDC/MPU 23 has a plurality of notch filtersconnected in series in the servo system of the head slider 12 (actuator16). The HDC/MPU 23 adapts the center frequency of one or a plurality ofthe notch filters in response to the oscillation frequency of theactuator 16. Specifically, the center frequency is matched to theoscillation frequency of the actuator 16. Thus, the oscillations of theactuator 16 which vary with the assembly variations of the HDD 1 or thesystem with an installed HDD 1 can be effectively suppressed.

When a read/write command is fetched from the host 51, the HDC/MPU 23starts a seek. The HDC/MPU 23 moves (seeks) the head slider 12 to thedata track (target data track) at the address indicated by the commandfrom the current radial position. The HDC/MPU 23 converts the addressspecified by the command into a servo address for specifying the targetradial position. When the seek ends, HDC/MPU 23 maintains the headslider 12 above the target data track (following).

In a seek or a following, HDC/MPU 23 uses the servo data read out fromthe recording surface to control the actuator 16 (VCM 15). Generally,seek control controls the actuator 16 (VCM 15) by velocity control andposition control which use the servo data. In following control, theHDC/MPU 23 controls the positioning so that the current radial position(servo address) of the head slider 12 and the target radial position(servo address) are within the specified range.

The servo sectors are formed at nearly equal intervals and separated inthe circumferential direction on the recording surface. Consequently,the head slider 12 reads out servo data at a constant period (servosampling period). HDC/MPU 23 controls the drive current of the VCM 15 inresponse to the position error signal which indicates the position errorof the current servo address indicated by the servo data and the targetservo address.

FIG. 2 is a block diagram modeling the servo control system in the HDD1, in accordance to an embodiment of the present invention. Each blockindicates a transfer function. In FIG. 2, a control target 31 is theservo control target of the HDC/MPU 23 and includes the motor driverunit 22, VCM 15, actuator 16, and head slider 12. The operation quantityof the control target 31 is the control data from the HDC/MPU 23 to themotor driver unit 22 and represents the drive current value of the VCM15. Feedback from the control target 31 is a signal (data) indicatingthe current radial position of the head based on the servo data read outby the head slider 12.

In this embodiment, the servo control system in the HDC/MPU 23 has amain servo controller 231, a fixed notch filter 232, an adaptive chasingnotch filter 233, an adaptive chasing peak filter 234, and a fixed peakfilter 235. The types and number of notch filters and peak filters areappropriately designed. FIG. 2 shows an example of the simplifiedstructure in order to describe this embodiment. For example, the HDC/MPU23 may have a plurality of fixed notch filters, a plurality of adaptivechasing notch filters, a plurality of adaptive chasing peak filters, anda plurality of fixed peak filters.

Alternately, there may not be adaptive chasing peak filters and onlyfixed peak filters. Typically, these function elements are implementedby hardware in the HDC/MPU 23, but a portion of the functions may beimplemented by the calculations of the MPU. In an embodiment, the notchfilters and the peak filters are built from hardware to implement theprocesses without delays.

The main servo controller 231 calculates the VCM current value (controldata representing the current) in response to the position error signal.Since the control by the main servo controller 231 is basicallyproportional-integral-derivative (PID) control, it is difficult tohandle large oscillations of the head slider 12 (actuator 16) whilemaintaining control stability. Therefore, HDC/MPU 23 has notch filters232, 233 arranged in series at the output of the main servo controller231 and peak filters 234, 235 connected in parallel to the main servocontroller 231.

The notch filters 232, 233 act to suppress the oscillations of theactuator 16. The components corresponding to the oscillation frequenciesof the actuator 16 in signals from the main servo controller 231 can bereduced, and large oscillations at the oscillation frequency of theactuator 16 can be suppressed. Typically, the maximum gain of the notchfilters is 1.0 (0 dB). The notch filters have the smallest gain at thecenter frequency. This gain increases as the frequency moves away fromthe center frequency. The filter shape is usually symmetric with respectto the center frequency, but may be asymmetric.

The filter shape of the fixed notch filter 232 is fixed (invariant).Specifically, the values for specifying the filter shape, such as thecenter frequency, the half width, and the gain at the center frequency,are fixed. The filter shape of the adaptive chasing notch filter 233 isvariable. In this embodiment, HDC/MPU 23 can set the center frequencyand the width (usually, the half width) in response to the oscillationsof the actuator 16. Values other than these can be set. The gain may bea function of the half width and/or the center frequency, or the gainmay be constant.

At least a portion of the oscillation frequencies (frequenciesexhibiting large oscillations) of the actuator 16 is known in thedesign. Therefore, fixed notch filters having center frequencies whichcorrespond to these oscillation frequencies can be prepared, and theoscillations of the actuator 16 can be effectively suppressed by asimple structure. An appropriate number of mounted fixed notch filtersis selected in accordance with the design of HDD 1.

The oscillations of the actuator 16 change depending on conditions suchas the assembly variations, the temperature, and the fixing state in thesystem having an installed HDD 1. Therefore, the adaptive chasing notchfilters 233 are installed in the HDD 1. The HDC/MPU 23 suppresses theoscillations of the actuator 16 by appropriately setting the filtercharacteristics of the adaptive chasing notch filter 233.

The peak filters 234, 235 act to suppress the oscillations of the headslider 12 (actuator 16) caused by noise. The noise includes repeatablerun-out (RRO) caused by disk eccentricity. The fixed peak filter 235knows the peak frequency in advance and operates so that the peakfrequency suppresses regular oscillations. The center frequency (peakfrequency) of the fixed peak filter 235 is fixed. Generally, the peakfrequency, the gain, and the filter shape are fixed. An appropriatevalue is selected for the number of mounted fixed peak filters based onthe design of the HDD 1.

In contrast, the adaptive chasing peak filter 234 has the action wherethe frequency caused by external oscillations suppresses the irregularoscillations. An appropriate number is selected for the number ofmounted fixed peak filters and adaptive chasing peak filters inaccordance with the design of the HDD 1. In addition, if not needed inthe design, one or both of these filters may be omitted.

HDC/MPU 23 generates the data (signal) indicating the current radialposition of the head slider 12 from the servo data read out by the headslider 12 included in the control target 31. HDC/MPU 23 holds the dataindicating the target radial position specified by a command from thehost 51. HDC/MPU 23 calculates the position error signal (data), whichis the difference between the target radial position and the currentradial position.

The main servo controller 231 conducts the specified calculation processfor the position error signal to calculate the VCM current value formoving the head slider 12 closer to the target radial position (suppressposition errors). The position error signal is also input to a pluralityof peak filters 234, 235 connected in parallel to the main servocontroller 231. Each of the peak filters 234, 235 has a filter shapewhich has a maximum gain at the peak frequency and a larger reduction inthe gain when moving away from the peak frequency. Therefore, thecharacteristic frequency component in the position error signal becomesthe filter output.

The outputs of the peak filters 234, 235 are added to the output fromthe main servo controller 231, and the total value (signal) is appliedto the notch filters 232, 233. Each of the notch filters 232, 233 isdesigned so that the minimum gain is at the center frequency, and thegain and the phase characteristics produce a stable system. Therefore,the servo gain becomes smaller at and near the center frequencies of thenotch filters and becomes stable, and the oscillations of the actuator16 stop.

The HDC/MPU 23 in this embodiment features controlling the adaptivechasing notch filter 233. The fixed notch filter 232 operates in theservo loop. In one embodiment, the adaptive chasing notch filter 233operates in the servo loop when oscillations start and is passed throughwhen oscillations do not start. Therefore, an unneeded notch filter isremoved from the servo loop, and the stability of servo control isimproved by not having the phase margin consumed by the unneeded notchfilter. Based on this design, the adaptive chasing notch filter 233 mayoperate in servo control.

HDC/MPU 23 of this embodiment references the internal variables of theadaptive chasing notch filter 233 and estimates the oscillationfrequency. FIG. 3 shows the adaptive chasing notch filter 233 and thefunction illustrating the update quantity of the center frequency.HDC/MPU 23 references the internal variables x,y. The shift quantity(update quantity) of the center frequency in response to the estimatedoscillation frequency is dB and is a function of the internal variablesx,y. The shift quantity dB indicates the increase or decrease in thecenter frequency, and is positive for an increase and negative for adecrease. Thus, the shift quantity dB includes a positive or a negativesign. As shown in FIG. 3, a notch filter can be represented by thefollowing equation.(K/(1−Ez⁻¹−Fz⁻²))×(1+Bz⁻¹+Cz⁻²)  (Equation 1)

The center frequency of the notch filter 233 is determined by B inequation 1. Namely, B is the value which represents the centerfrequency. HDC/MPU 23 uses the input x and the output y of the numerator(1+Bz⁻¹+Cz⁻²) in equation 1 to estimate the oscillation frequency.Specifically, HDC/MPU 23 estimates the oscillation frequency bydetermining the magnitude relationship between the current centerfrequency of the notch filter 233 and the oscillation frequency from theinternal variables x,y. The center frequency changes in response to thisestimate.

This frequency estimation method and the method for updating the filterfrequency are similar to the methods used in the adaptive chasing peakfilter 235. The peak filter and the notch filter can be represented bythe same general equation as a two-dimensional IIR filter. The valuerepresenting the center frequency appears in the denominator of the peakfilter and in the numerator of the notch filter.

However, because the notch filter has different characteristics than thepeak filter in the servo system, the frequency-adaptive method in thepeak filter cannot be applied without modification to the notch filter.One reason is the sampling rate difference between the notch filter andthe adaptive filter. Another reason is the presence of DC components inthe input and the output of the notch filter. Yet another reason is thedifference in the source of the oscillations. The methods which respondto the differences between the notch filter and the adaptive filter aredescribed below for the adaptation of the adaptive chasing peak filter235.

The description begins with the solution method for the problems of thedifference in the sampling rates and the presence of DC components.Generally, the resonance frequency of the mechanical resonance mode ofthe actuator 16, which is the target of the notch filter, includesfrequencies higher than the servo sampling frequency. Therefore, in atypical servo system for an HDD, the operating frequency of the notchfilter is higher than the servo sampling frequency. Specifically, theoperating frequency of the notch filter is an integral multiple (integerof 2 or more) of the servo sampling frequency.

Thus, the notch filters 232, 233 operate at the multiple sampling rate.On the other hand, in the servo system, other elements which aredifferent than the notch filters, such as the main servo controller 231and the peak filters 234, 235, operate at a single sampling rate.Therefore, depending on the design, HDC/MPU 23 can reference theinternal variables x,y only at the single sampling rate and cannotreference at the multiple sampling rate.

In addition, the inputs and outputs of the notch filters 232, 233 aredata showing the drive currents of the VCM 15. The values of theseinputs and outputs include DC components. Since a bias force is appliedto the actuator 16, a drive force to resist the bias force is generatedby the VCM 15.

In the estimation of the oscillation frequency by using the internalvariables x,y, HDC/MPU 23 removes the abovementioned DC components. WhenDC components exist, HDC/MPU 23 mistakenly recognizes the start ofoscillations at the frequency of the DC components, that is, 0 Hz, andmatches the adaptive chasing notch filter 233 to 0 Hz.

The HDC/MPU 23 in this embodiment references at the single sampling ratethe internal variables x,y of the adaptive chasing notch filter 233operating at the multiple sampling rate, and uses the referencedinternal variables x,y to estimate the oscillation frequency.Furthermore, HDC/MPU 23 removes the abovementioned DC components in thecalculation of the internal variables x,y in the estimation of theoscillation frequency. For example, by using a simple calculation,HDC/MPU 23 estimates the oscillation frequency at the single samplingrate and removes the abovementioned DC components.

HDC/MPU 23 can determine the shift quantity dB(n) for the centerfrequency of the adaptive chasing notch filter 233 by using thefollowing Equation 2.dB(n)=ΔB×x(n−1)×y(n),  (Equation 2)

where n is the multiple rate sampling time; and ΔB is a constant, or afunction of internal variables x and y, or a function of othervariables.

When the magnitude relationship of the center frequency of the notchfilter 233 and the oscillation frequency is an inverse relationship, thesign of x(n−1)×y(n) changes. HDC/MPU 23 can use this characteristic toadjust the center frequency to match the center frequency to theoscillation frequency, and identify the oscillation frequency.

Equation 2 uses the internal variables x,y at the multiple samplingrate. Consequently, when HDC/MPU 23 can reference the internal variablesx,y only at the single sampling rate, the calculation in Equation 2above cannot be used. In this structure, HDC/MPU 23 determines the shiftquantity dB (m) in accordance with the following Equation 3.dB(m)=ΔB×((1−z ^(−M))×x(m−1))×((1−z ^(−M))×y(m))  (Equation 3)

Variable m is the single sampling rate time. The multiple sampling rateis M times the single sampling rate, m=M×n. Variable M is an integer ofat least 2 and is a constant number. The “−1” in x(m⁻¹) denotes a delayof one sample at the multiple sampling rate. The variable z^(−M) is(z⁻¹)^(M). The operator z⁻¹ is a one-sample delay at the multiplesampling rate.

The quantity dB(m) represents the shift quantity of the center frequencycalculated from the internal variables x,y, which were referenced at thesingle sampling rate timing. The shift quantity dB(m) of the centerfrequency can be regarded as an estimate of the difference between thecurrent center frequency and the oscillation frequency. HDC/MPU 23repeatedly estimates the oscillation frequency by using Equation 3 andupdates the center frequency, and the center frequency graduallyapproaches the oscillation frequency.

When m is replaced by M×n in Equation 3 above, the following Equation 4can be obtained.dB(m)=ΔB×((1−z ^(−M))×x(M×n−1))×((1−z ^(−M))×y(M×n))=(1−z ^(−M))²×ΔB×x(M×n−1)×y(M×n)  (Equation 4)

Equation 4 can be represented by the following Equation 5.dB(n)=(1−z ^(−M))² ×ΔB×x(n−1)×y(n), If n=mdB(n)=0, If n≠m  (Equation 5)

The differences between Equation 5 and Equation 2 are the term(1-z^(−M))² in Equation 5 and the sampling times for fetching theinternal variables x,y. Since the result of the (1−z^(−M))² calculation(filter) is positive, Equation 5 can be used in the estimation of theoscillation frequency, similar to Equation 2. The calculation byEquation 5 can use only the internal variables x,y values referenced atthe single sampling rate timing and can determine the shift quantity ofthe center frequency (estimate of the center frequency).

The adaptive chasing notch filter 233 is a two-dimensional filter.HDC/MPU 23 can obtain one output internal variable y(m) and two inputinternal variables x(m),x(m−1) at the single sampling rate times. Thevalue of x(m−1) is the internal variable x delayed by one sample fromx(m) at the multiple sampling rate. As shown in Equation 3, HDC/MPU 23can use the internal variables x(m−1),y(m) referenced at the singlesampling rate timing to estimate the oscillation frequency.

Equation 3 calculates the product of ((1−z^(−M))×x(m−1)) and((1−z^(−M))x y(m)). The operator z^(−M) is a delay of M samples at themultiple sampling rate. That is, this operator is delayed by one sampleat the single sampling rate. Equation 3 calculates the difference((1−z^(−M))×x(m−1)) between the values of internal variable x atdifferent sampling times and the difference ((1−z^(−M))×y(m)) betweenthe values of internal variable y at different sampling times. Thesesubtraction processes remove the DC components from the internalvariables x,y.

From the calculation process according to Equation 3, HDC/MPU 23 canreference the internal variables of the adaptive chasing notch filter233 operating at the multiple sampling rate and estimate the oscillationfrequency (and the update of the center frequency). In addition, fromthe calculation process according to Equation 3, HDC/MPU 23 can removethe DC components from the internal variables x,y and appropriatelyestimate the oscillation frequency.

Equation 3 calculates the differences of the internal variables offsetby one multiple rate sample. HDC/MPU 23 may use the difference of theinternal variables offset by only 2 or more multiple rate samples.Namely, z−^(kM) may be used in place of z^(−M), where k is an integer ofat least 2. However, in one embodiment, the DC components are removed bycalculating the differences of the internal variables offset by onemultiple rate sample as in the calculation in Equation 3 because thecalculation is simpler and more accurate.

From the calculation of Equation 3, HDC/MPU 23 can reference at thesimple sampling rate the internal variables of the adaptive chasingnotch filter 233 operating at the multiple sampling rate by using asimple calculation (filter structure), remove the DC components, andestimate the oscillation frequency.

As described above, in the adaptation of the adaptive chasing notchfilter 233, it is important to note that the suppression target is theoscillation of the actuator 16. The oscillations caused by the actuatoroscillations are eliminated when the oscillation conditions are removed.When the adaptive chasing notch filter 233 approaches the oscillationfrequency, the skirt of the filter overlaps the oscillation frequencyand the oscillation stops. HDC/MPU 23 cannot estimate higher oscillationfrequencies. Thus, the offset between the center frequency of the notchfilter 233 and the oscillation frequency becomes larger. As a result ofthe offset, the filter width widens without necessarily becoming thespecified minimum value.

As shown in the flow chart in FIG. 4, in one embodiment, HDC/MPU 23inserts the adaptive chasing notch filter 233 in the servo loop (S12) ifthe oscillations start (Y in S11). If the oscillations do not start (Nin S11), the notch filter 233 is set to pass through.

When the notch filter 233 is passed through, the notch filter 233 is notin the servo loop (without the filter calculation, the unchanged inputbecomes the output), or the coefficients are given so that thecharacteristic of the notch filter 233 becomes equivalent to the statewhere the notch filter is not in the servo loop (filter calculationwhere the input and the output become equal). The presence or absence ofoscillations can be determined by using position error signal (PES).

After the adaptive chasing notch filter 233 is inserted in the servoloop, HDC/MPU 23 adapts the notch filter 233. Specifically, theoscillation frequency is estimated, and the center frequency of thenotch filter 233 is matched to that frequency. The adaptive chasingnotch filter 233, which is inserted in the servo loop, has a widthwithin the specification (half width) in the initial state. In oneembodiment, that width is zero. When the width is zero, the notch filter233 does not exhibit the action of a filter in servo loop state.

HDC/MPU 23 estimates the oscillation frequency of the notch filter 233while maintaining the width within the specification (while holding atzero) (S13). As described above, HDC/MPU 23 references the internalvariables of the notch filter 233 at the single sampling rate timing,and uses the x(m−1)×y(m) to repeatedly estimate the oscillationfrequency and update the center frequency. When the center frequencyconverges at the oscillation frequency (Y in S14), HDC/MPU 23 widens thewidth of the notch filter 233 (S15). The center frequency whichconverges at the oscillation frequency is within the specified rangefrom the oscillation frequency. Specifically, the difference between thecenter frequency and the oscillation frequency is within a preset range.

Thus, the oscillation frequency can be estimated while the width of theadaptive chasing notch filter 233 is maintained within the specifiedvalues, and the width widens after the estimation ends, and the centerfrequency of the adaptive chasing notch filter 233 can approach closerto the oscillation frequency.

An estimation method of the oscillation frequency in step S13 isspecifically described with reference to the flow chart in FIG. 5.HDC/MPU 23 uses dS(m) which is represented by the following Equation 6.dS(m)=((1−z ^(−M))×x(m−1))×((1−z ^(−M))×y(m))  (Equation 6)

The right side of Equation 6 is ΔB removed from the right side ofEquation 3.

If the absolute value |dS(m)| of dS(m) is greater than the specifiedvalue dS_(min) (Y in S131), HDC/MPU 23 continues to estimate thefrequency and calculate the center frequency update. Under thiscondition, the oscillations of the actuator 16 continue. When |dS(m)| isless than dS_(min) (N in S131), the frequency estimation process is notconducted. Next, HDC/MPU 23 determines the sign of dS(m) (S132). IfdS(m) is less than 0 (Y in S132), the oscillation frequency is less thanthe center frequency. Conversely, if dS(m) is greater than 0 (N inS132), the oscillation frequency is greater than the center frequency.In the notch filter represented by Equation 1, the center frequencydecreases as the value of coefficient B increases.

The case when the center frequency is less than the oscillationfrequency is described below. Namely, this is the case when dS(m) isless than 0 (Y in S132). HDC/MPU 23 calculates the update quantity dB ofthe center frequency and updates the center frequency (S133). The updatequantity dB of the center frequency is the value of dBinc added to theprevious update quantity. The dBinc is a positive number and a constantnumber. The initial value of the update quantity dB of the centerfrequency is the specified value, for example, zero.

Next, HDC/MPU 23 determines the sign of dB (S134). When dB is less than0 (Y in S134), HDC/MPU 23 sets dB to 0 (S136). In this example, since dBis greater than or equal to 0 (N in S134), HDC/MPU 23 maintains thevalue calculated in step S133 as dB. HDC/MPU 23 maintains dB lower thanthe specified dBmax (S136, S137). The dBmax value is the positivespecified value.

HDC/MPU 23 determines whether dB is 0 or not (S138), if not 0 (N inS138), the coefficient B is updated. The series of processes is repeatedfor each servo at the single sampling rate. After the previous centerfrequency is updated, the center frequency is less than the oscillationfrequency (Y in S132). HDC/MPU 23 adds dBinc to the update quantity ofthe previous center frequency and calculates the current update quantityof the center frequency (S133). Thus, the update quantity of the centerfrequency increases in response to the number of times that the sign ofdB continues. As described above, dB is maintained below dBmax (S136,S137). Since dB is positive (N in S134), HDC/MPU 23 maintains the valuecalculated in step S133 as dB.

Next, HDC/MPU 23 determines whether dB is 0 or not (S138). Since dB isnot 0 (N in S138), HDC/MPU 23 updates the coefficient B (S145). Byrepeating steps S132 to S138 for each servo, the center frequencyapproaches the oscillation frequency and becomes the larger value. Atthis time, dS(m) is greater than 0 (N in S132). HDC/MPU 23 subtractsdBinc from the previous update quantity dB of the center frequency anddetermines the current update quantity (S139).

HDC/MPU 23 determines the sign of dB (S140). In this example of theprocess, since dB is greater than 0 (Y in S140), HDC/MPU 23 sets dB to 0(S141). Since dB is greater than or equal to dBmin (N in S142), HDC/MPU23 maintains dB at 0 (step S143 is skipped). The value of dBmin is thenegative specified value.

HDC/MPU 23 determines whether dB is 0 or not (S138). Since dB is 0 (Y inS138), HDC/MPU 23 calculates the width W (S144). In the example of theprocess described above, a center frequency smaller than the oscillationfrequency gradually increases and finally exceeds the oscillationfrequency. At this time, the center frequency and the oscillationfrequency can be regarded as roughly matched (a difference in thedesired range). Therefore, HDC/MPU 23 calculates the width W. When thecenter frequency roughly matches the oscillation frequency, the sign ofdS(m) changes frequently. Each time the sign changes, dB=0, and thewidth W is frequently calculated instead of the frequency estimationprocess. Namely, after the center frequency converges at the oscillationfrequency and the frequency estimation process ends, the process forcalculating the width W is conducted.

When the initial value of the center frequency is greater than theoscillation frequency, the processes by the HDC/MPU 23 become thereverse of the processes described above. HDC/MPU 23 repeats steps S132to S143 and step S138 to decrease the center frequency (S139). Thisupdate quantity dB is maintained in the range above dBmin (S142, S143).If the center frequency is less than the oscillation frequency (Y inS132), dB=0, and HDC/MPU 23 calculates the width W (S144). When thecenter frequency roughly matches the oscillation frequency, the sign ofdS(m) changes frequently, and dB=0 each time. Therefore, the width W iscalculated frequently instead of the frequency estimation process.Namely, after the frequency estimation process essentially ends, theprocess for calculating the width W is conducted.

As described above, the adaptive chasing notch filter 233 is calculatedonly when HDC/MPU 23 is |dS(m)|>dS_(min) (Y in step S131). When|dS(m)|<dS_(min), since the absolute value of dS(m) is small, the signof dS(m) is easily changed by noise. Therefore, HDC/MPU 23 updates Bonly when |dS(m)| is larger. After the oscillations end, the offset ofthe B value caused by noise and resumption of the oscillations areprevented. Thus, when the oscillations stop, |dS(m)|<dS_(min), strayingof the notch filter 233 is prevented.

In the frequency estimation calculation, the absolute value of the shiftquantity dB of the center frequency increases in response to the numberof times that the dS sign continues. Thus, the absolute value of dBvaries, and the convergence characteristic of the center frequency tothe oscillation frequency is improved. In addition, since the variationrange of the shift quantity dB (maximum value of the absolute value ofthe shift quantity) is specified by dBmax and dBmin, the centerfrequency can be prevented from having a large offset from theoscillation frequency caused by an excessive shift.

As described above, from the |dS(m)|>dS_(min) condition, the dS value isdifficult for noise to affect. Therefore, the sign of dS changes onlywhen the magnitude relationship of the oscillation frequency and thecenter frequency of the notch filter 233 changes. At this time, thedifference between the oscillation frequency and the center frequency isa small value within the desired range and can be regarded as matching.Thus, after the oscillation frequency is essentially identified and theestimation process ends, HDC/MPU 23 starts the calculation of the halfwidth W of the notch filter 233.

In one embodiment, a method for calculating the half width W of thenotch filter 233 is described below with reference to the flow chart inFIG. 6. HDC/MPU 23 sets the value of the width W to the desired valuewhich can gradually increase beginning from the W value in the frequencyestimation and stop the oscillations. In one embodiment, the initialvalue of W is 0. In addition, In one embodiment, HDC/MPU 23 updates thevalue of the width W in the servo sampling. Specifically, HDC/MPU 23adds the update quantity dW to the current width W in the servosampling. The update quantity dW is a positive or negative value andincludes the sign.

As shown in FIG. 6, in one embodiment, HDC/MPU 23 uses PES to judgewhether there are oscillations. In this example, HDC/MPU 23 uses thevariance of PES (PESsigma²). Specifically, if PESsigma²>PES2_active (Yin S441), the judgment of HDC/MPU 23 is that oscillations are starting(are not stopping). PES2_active is the specified constant.

At this time, HDC/MPU 23 sets the update quantity dW of the width W todWinc (S442). dWinc is a positive constant. dWinc may be a function ofsome variable. In this case, the value of the function is a positivenumber. HDC/MPU 23 may judge whether oscillations are present or notfrom the function value of PES which is different than the PES variance,or the magnitude of the actuator oscillations.

If PESsigma² is less than PES2_active (N in S441), the judgment ofHDC/MPU 23 is that the oscillations are stopping. However, if thePESsigma² is a value close to PES2_active, in one embodiment, the widthW in the available range is larger, and the margin is maintained in thenotch filter.

When PESsigma²>PES2_Wup (Y in S443), if the width W is less than Wmin (Yin S444), HDC/MPU 23 sets the update quantity dW to dWinc (S442).Namely, HDC/MPU 23 increases the width W so that the width approachesWmin. Wmin can be the value of the specified constant added to the widthW which initially satisfied a function of the center frequency or thecondition of step S441. Thus, by increasing the width W to approachWmin, the margin can be maintained by the notch filter.

When the width W is greater than Wmin (N in S444), HDC/MPU 23 sets theupdate quantity dW to 0, namely, the width W does not change and ismaintained (S445). The causes of the useless increase in the width W arethe decrease in the phase margin in the servo system and the drop inservo performance.

When the PES2_Wup is ≧PESsigma² (N in S443), the margin for preventingoscillations is guaranteed. Consequently, HDC/MPU 23 maintains the widthW and does not increase its value (S445), or decreases the value (S448).dWdec in step S448 is a negative constant. dWdec may be a function ofsome variable. In this case, the value of the function is a negativenumber.

Specifically, when PES2_Wup≧PESsigma²>PES2_Wdown (N in S443 and N inS446), HDC/MPU 23 sets dW to 0 and maintains the current width W (S445).PES2_Wdown in S446 is a positive number no more than PES2_Wup and is anumber which differs from or is identical to PES2_Wup.

When PES2_Wup≧PESsigma², the oscillations of the actuator 16 aresuppressed. However, when PESsigma²>PES2_Wdown, the decrease in thewidth W of the notch filter 233 increases the probability ofoscillations of the actuator 16 starting. Consequently, in oneembodiment, HDC/MPU 23 sets dW to 0 and maintains the current width W(S445).

When PESsigma²<PES2_Wdown (Y in S446) and W>Wmin (Y in S447), HDC/MPU 23sets dW to dWdec to decrease the width W (S448). WhenPESsigma²<PES2_Wdown, the width W of the notch filter 233 has sufficientmagnitude. Furthermore, when W>Wmin, HDC/MPU 23 decreases the width W sothat the width approaches Wmin. Thus, the phase margin in the servosystem can be increased, and the servo system can be more stable.

When W of the notch filter 233 is less than Wmin (N in S447), the widthW is sufficiently small in order to ensure the phase margin of the servosystem. Therefore, as in this example of the structure, in order to morereliably prevent the oscillations of actuator 16, in one embodiment,HDC/MPU 23 sets dW to 0 and maintains the current width W (S445).

If the width W of the notch filter 233 is too large, the servo systembecomes unstable. Consequently, the width W is set to the maximum valueWmax. HDC/MPU 23 conducts the processes described with reference to FIG.6 in the servo sampling and determines the update quantity of the widthW. HDC/MPU 23 adds the update quantity dW to the current width W, but ifthat value is greater than Wmax, the width W is set to Wmax.

In one embodiment, the maximum value Wmax is a function of the centerfrequency. When the maximum value Wmax is constant regardless of thecenter frequency, the phase delay quantity is changed by the centerfrequency. Therefore, the notch filter 233 is difficult to generate sothat the phase margin of the servo system becomes larger than thespecified value. When the maximum value Wmax is a function of the centerfrequency, the HDC/MPU 23 can give the maximum to the phase delayquantity due to the generated notch filter 233. Therefore, the phasemargin of the servo system can easily be held in a larger range than thespecified values.

Next, the method for generating the adaptive chasing notch filter 233 isexplained. From the method described above, the center frequency and thewidth (half width) of the notch filter 233 are determined. HDC/MPU 23generates the notch filter 233 from these values and the gain at thecenter frequency. As described above, the notch filter 233 can berepresented by Equation 1.

HDC/MPU 23 generates each coefficient K, E, F, B, C of the notch filter233 from the B which represents the center frequency, the width W, andthe depth D (gain at the center frequency). In one embodiment, the depthD is given by a constant number. Each coefficient can be generated bythe following equations.K=1−W  (Equation 7)C=1−2×D  (Equation 8)E=−K×B  (Equation 9)F−C+2×W  (Equation 10)

The depth D is the gain at the center frequency of the notch filter andsatisfies the following condition. When W is 0, the characteristic ofthe notch filter 233 is pass through.1>>D≧0  (Equation 11)

In addition, because the maximum Wmax of the width W is defined by afunction of the center frequency, the width W satisfies the followingcondition.1>>Wmax(B)≧W≧0  (Equation 12)

When either the width W or the depth D is 0, the other is a valuegreater than 0. Thus, the gain characteristic until the internalvariable is prevented from becoming infinity.

An example of the function (function of B) of the center frequency ofwidth W is the following function.Wmax(B)=w0+w1×(B0−B),B<B0;Wmax(B)=w0,B≧B0,  (Equation 13)

where 1>>w0≧0 holds, and w0, w1, and B0 are constants.

In accordance with the preceding description, HDC/MPU 23 can generate anappropriate adaptive chasing notch filter 233 corresponding to theoscillation frequency of the actuator 16. The adaptive chasing notchfilter 233 implemented in the HDD 1 as a product can effectivelysuppress the oscillations of the actuator 16 which were generated inresponse to the state of the system implementing the filter or theenvironmental changes.

In the manufacturing steps, HDD 1 can use the adaptive chasing notchfilter 233. In the testing steps in the manufacturing of the HDD 1,HDC/MPU 23 stops the oscillations of the actuator 16 by adapting theadaptive chasing notch filter 233. HDC/MPU 23 generates a new fixednotch filter (time-invariant notch filter) based on the center frequencyand the width of the adaptive chasing notch filter 233 and adds thefilter to the servo system. Therefore, the notch filter required becauseof the fluctuations of each drive can be set individually.

For example, HDD 1 has one or a plurality of fixed notch filters sharedby all HDDs having the same design, one or a plurality of fixed notchfilters having individual settings, and an adaptive chasing notch filter233. The notch filters having individual settings are passed throughbefore being set. As described above, HDC/MPU 23 determines thecoefficients of the adaptive chasing notch filter 233 which stops theoscillations of the actuator 16. HDC/MPU 23 determines the coefficientsfor a notch filter having individual settings from the determinedcoefficients and sets these coefficients in the notch filter havingindividual settings. Thus, the fixed notch filter is inserted in theservo system.

In one embodiment, HDC/MPU 23 sets a width W greater than the width W ofthe adaptive chasing notch filter 233, which stopped the oscillations,in the notch filter having individual settings. Thus, the margin isensured. In addition, in the frequency band which should ensure the gainand margin, HDC/MPU 23 increases the portion corresponding to the marginwhich should ensure the gain of the frequency band in the filter forpromoting oscillations. HDC/MPU 23 removes the filter for promotingoscillations after the notch filter having individual settings is set.Thus, the margin which should be guaranteed can be reliably guaranteed.

The region where the notch filter is added changes depending on thedesign of the HDD. Various embodiments of the methods can be applied todisk drives other than HDDs, and it does not matter whether or not thedisk is fixed in a cabinet. It is important for the center frequency ofthe notch filter to be in the vicinity of the oscillation frequency ofthe actuator, but the center frequency is not necessarily limited tomatching.

Various embodiments of the present invention are thus described. Whilethe present invention has been described in particular embodiments, itshould be appreciated that the present invention should not be construedas limited by such embodiments, but rather construed according to thefollowing claims.

The invention claimed is:
 1. A disk drive comprising: a head foraccessing a disk, an actuator for moving said head in a radial directionof said disk, and a controller for servo control of said actuator basedon servo data read out from said disk by said head, wherein saidcontroller is configured for maintaining a width of an adaptive chasingnotch filter below a specified value in servo system control andestimating an oscillation frequency of said actuator, wherein saidcontroller compares a magnitude of a value in a calculation of saidestimate of said oscillation frequency to a threshold, and estimatessaid oscillation frequency and updates a center frequency of saidadaptive chasing notch filter when said value is greater than saidthreshold, and wherein said controller compares a function valuerepresenting said oscillations of said actuator and a first threshold tojudge whether said oscillations have stopped, and increases said widthwhen said function value is between a second threshold which is lessthan said first threshold, and a value of said width of the adaptivechasing notch filter which stops said oscillations is less than a Wminvalue, wherein said Wmin value is greater than said value of said widthwhich stops oscillations to said width of said adaptive chasing notchfilter.
 2. The disk drive of claim 1, wherein said controller is furtherconfigured for moving a center frequency of said adaptive chasing notchfilter closer to said oscillation frequency.
 3. The disk drive of claim1, wherein said controller is further configured for increasing anupdated quantity of said center frequency in response to a number ofconsecutive updates of said center frequency.
 4. The disk drive of claim3, wherein a maximum value for a absolute value of said update quantityof said center frequency is specified.
 5. The disk drive of claim 1,wherein said controller uses a function value representing saidoscillations of said actuator, when a position error signal is thevariable, to judge whether said oscillations of said actuator havestopped, and said width of said adaptive chasing notch filter isdetermined from a value of said width for stopping said oscillations. 6.The disk drive described in claim 1, wherein said controller decreasessaid width when said function value is less than a third threshold whichis less than said second threshold, and the value of said width isgreater than said value Wmin.
 7. The disk drive described in claim 1,wherein a maximum value of said width is specified, and said controllerdetermines said width so that said width is less than said maximumvalue.
 8. The disk drive of claim 7, wherein said maximum value isrepresented by a function of said center frequency of said adaptivechasing notch filter.
 9. The disk drive of claim 1, wherein saidcontroller uses filter coefficients determined by adaptation of saidadaptive chasing notch filter to set a new fixed notch filter.
 10. Thedisk drive of claim 1, wherein said adaptive chasing notch filteroperates at a multiple sampling rate.